Comparison of Powertrain Configuration Options for Plug-in HEVs from a Fuel Economy Perspective

The first commercially available plug-in hybrid electric vehicle (PHEV), the General Motors (GM) Volt, was introduced into the market in mid-December 2010. The Volt uses a series-split powertrain architecture, which provides benefits over the series architecture that typically has been considered for use in electric-range extended vehicles (EREVs). A specialized EREV powertrain, called the Voltec, drives the Volt through its entire range of speed and acceleration with battery power alone and within the limit of battery energy, thereby displacing more fuel with electricity than a PHEV, which characteristically blends electric and engine power together during driving. This paper assesses the benefits and drawbacks of these two different plug-in hybrid electric architectures (series versus series-split) by comparing component sizes, system efficiency, and fuel consumption over urban and highway drive cycles. Based on dynamic models, a detailed component control algorithm was developed for each PHEV. In particular, for the GM Voltec, a control algorithm was proposed for both electric machines to achieve optimal engine operation. The powertrain components were sized to meet all-electric-range, performance, and grade capacity requirements. This paper presents and compares the impact of these two different powertrain configurations on component size and fuel consumption. INTRODUCTION Plug-in hybrid vehicles (PHEVs) combine an internal combustion engine (ICE) and an electrical energy/power source that is composed of a battery and one or two electric machines. Compared with the common hybrid electric vehicle (HEV), a PHEV has greater potential to improve fuel efficiency and reduce emissions, since it allows full electric driving and can obtain electric power easily from the home electricity grid [1]. The PHEV is also competent for long-distance driving with its HEV function. The Volt is manufactured by the Chevrolet division of General Motors (GM). This sedan-type PHEV was rated in model-year 2011 by the U.S. Environmental Protection Agency. According to GM, the Volt can travel 20 to 50 miles on its lithium-ion battery of 16 kWh. The GM Voltec powertrain architecture provides four modes of operation, including two that are unique and maximize the Volt’s efficiency and performance. The electric transaxle has been specially designed to enable patented operating modes both to improve the electric driving range when operating as a battery electric vehicle and to reduce fuel consumption when extending the range by operating with an ICE. The Voltec powertrain introduces a unique, two-motor electric vehicle (EV) driving mode that allows both the driving motor and the generator to provide tractive effort while simultaneously reducing electric motor speeds and the total associated electric motor losses. For HEV operation, the Voltec transaxle uses the same hardware that enables one-motor and two-motor operation to provide the completely decoupled action of a pure series hybrid, as well as a more efficient flow of power with decoupled action for driving at light load and high vehicle speed [2, 3]. When designing a vehicle for a specific application, the goal is to select the powertrain configuration that maximizes the fuel displaced and yet minimizes the sizes of components. The power-split system is the most commonly used system in currently available hybrid vehicles. However, the design of the power-split system for the PHEV is based on the blended strategy, and it has a relatively short electric driving range [4]. The series configuration for the PHEV, on the other hand, is often considered to be closer to a pure electric vehicle when compared with a split configuration. In this study, a comparative analysis is conducted on the Voltec and a pure series for the PHEV. Two vehicle powertrain configurations are sized to achieve similar performance for all-electric range (AER) approaches based on midsize vehicle applications. The component sizes and the fuel economy of each option are examined.